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Research on Strong Ground Pressure of Multiple-Seam Causedby Remnant Room Pillars Undermining in Shallow Seams

Dan Yu 1,2 , Xiaoyong Yi 2,* , Zhimeng Liang 2 , Jinfu Lou 3 and Weibing Zhu 1,2,*

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Citation: Yu, D.; Yi, X.; Liang, Z.;

Lou, J.; Zhu, W. Research on Strong

Ground Pressure of Multiple-Seam

Caused by Remnant Room Pillars

Undermining in Shallow Seams.

Energies 2021, 14, 5221. https://

doi.org/10.3390/en14175221

Academic Editor: Adam Smolinski

Received: 4 August 2021

Accepted: 21 August 2021

Published: 24 August 2021

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1 State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology,Xuzhou 221116, China; ts20020179p21@cumt.edu.cn

2 School of Mines, China University of Mining and Technology, Xuzhou 221116, China;liangmeng1019@163.com

3 China Coal Technology and Engineering Group Co., Ltd., Coal Mining Research Institute,Beijing 100013, China; loujinfu@tdkcsj.com

* Correspondence: TS18020059A3TM1@cumt.edu.cn (X.Y.); cumtzwb@cumt.edu.cn (W.Z.)

Abstract: Numerous room-and-pillar mining goaf are apparent in western China due to increasingsmall coal mining activities, which causes the collapse of the overlying coal pillars and the occurrenceof strong ground pressure on the longwall face and surface subsidence. In this study, Yuanbao BayCoal Mine, Shuozhou, Shanxi, was selected to study the collapse of the overlying coal pillars on thelongwall face and reveal the mechanism of the pillar collapse and the disaster-causing mechanismcaused by strong ground pressure. Results show that the dynamic collapse process of coal pillars isrelatively complicated. First, the coal pillars on both sides of the goaf are destroyed and destabilized,followed by the adjacent coal pillars, which eventually cause a large-scale collapse of the coal pillars.This results in a large-scale cut-off movement of the overlying strata, and the large impact load thatacts on the longwall face causes an unmovable longwall face support. Moreover, the roof weightingis severe when strong ground pressure occurs on the longwall face, causing local support jammedaccidents. Furthermore, the data of each measurement point of the strata movement inside the groundborehole significantly increases, and the position of the borescope peeping error holes in the grounddrill hole rise steeply. The range of movement of the overlying strata increases instantaneously, andthe entire strata begin to move. Research on the mechanism of strong ground pressure can effectivelyprevent mine safety accidents and avoid huge economic losses.

Keywords: undermining; room pillar; multiple-seam; dynamic load pressure; pillar collapse; stratamovement

1. Introduction

Coal, which is one of the main energy sources of China, plays a significant role inChina’s energy consumption structure. Particularly, western China became the concen-trated place of numerous and increasing mining activities in the past decades. Most miningoperations in these regions occur in shallow-buried and close-distance coal seams [1].Existing and upper coal seams with simple geological conditions have been mined due tothe intensive mining activities in recent years, and the mining process of the close-distancelower coal seam has been inevitably affected in secondary mining. This multiple-seammining [2–5] causes an increase in the overburden movement range and surface damage.Moreover, the abnormal stress field and movement of the overlying strata in the stopecauses the appearance of abnormal ground pressure, resulting in the increasing attentionthat close-distance coal seam mining has received in recent years [6,7].

The main problem in the Yuanbao Bay Mine in Shuozhou, Shanxi, is ensuring thestability of the overlying coal pillars affected by undermining [2,3,8–12]. Several theoreticaland numerical studies have been conducted to address this problem and evaluate the sta-bility of coal pillars, including the modeling and calculation of coal pillar bearings [13–15],

Energies 2021, 14, 5221. https://doi.org/10.3390/en14175221 https://www.mdpi.com/journal/energies

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determination of the mechanical characteristics of coal pillars under long-term bearing ac-tion [16–18], investigation of the function evaluation method of the coal pillar stability [19],and the identification of the main factors affecting the stability of coal pillars [20,21].

The overlying coal seam goaf is affected by lower seam undermining if it is a room-and-pillar mined-out area. Moreover, the coal pillar collapse mechanism, the influence ofthis collapse on the working face of the lower coal seam, and the surface subsidence [22]when the overlying coal pillar loses stability have been extensively studied by severalexperts and scholars.

Therefore, the large-area support crushing is crucial and necessary due to the move-ment and instability of the overburden structure. Some scholars systematically analyzedthe large-area support crushing event of stope in China and found that the articulated rockblock structure formed by the key strata of the overburden lacks stability [23] and causesthe large-area support crushing event in a stope. Therefore, the working resistance [24] ofthe working face support should be at a high level to prevent support crushing. Meanwhile,Wang et al. [25] investigated the principles behind the destruction of loose confined aquifersin mining and revealed the important role of the unconsolidated confined aquifer (UCA) inthe process of load transfer due to its liquidity and timely replenishment [26]. On the otherhand, Huang et al. [27] focused on the number of key strata in shallow-buried and close-distance coal seams and determined that different overlying strata structures were formedafter one or two layers were broken. Additionally, the structure formed by one brokenlayer was carried together in the form of hinged arches and arch shells, while the structureof two broken layers was formed in a variety of combinations of steps and masonry, andthe composite overburden structure jointly bears the load of the overburden. Zhu et al. [28]studied the roof damage in room-and-pillar mined-out areas caused by longwall mining inthe lower coal seam and observed that different width-to-height ratios of coal pillars lead todifferent modes of collapse failure [29] and provided reference values for the developmenttrend of the cracks after collapse of different width-to-height ratios of coal pillars duringmining underneath the goaf. Wang et al. [30,31] have done great research on automaticallyformed roadway (AFR) and found that the new mining method satisfies the surroundingrock control requirement, which improves the safety and economic impact of coal mining.Feng et al. [32], using the key stratum theory, put forward the concepts of key stratumbreaking distance and advancing mining coal pillars to address the problem of coal seammining above the knife-pillar goaf and formed the knife-pillar goaf overmining [33–35]feasibility judgment theory and method. Wang et al. [36] comprehensively considered thefactors influencing the stability of the interburden and adopted the safety factor method toevaluate the stability of the interburden in order to ensure the safe mining of a coal seamthat overlies the knife-pillar goaf area. Dychkovskyi R. et al. [37] conducted research intostress-strain state of the rock mass condition in the process of the operation of double-unitlongwalls in order to substantiate changes in stress-strain state of rock mass in the processof long-pillar mining with the help of double-unit longwalls while evaluating stress of amine field in terms of Lvivvuhillia SE mine.

However, most of the existing research is only aimed at the collapse of coal pillargroups under static load. There is limited research on the stability of coal pillars and thedynamic collapse process of coal pillar groups under the influence of mining. Additionally,there is only one layer of the room-and-pillar mined-out region in overburden, which can-not show the dynamic collapse process. This limits the investigation into the combinationcollapse of the coal pillar and the roof during the dynamic collapse process. Meanwhile,new measurement method is applied to the strata movement, optical fiber is employedduring strata movement monitoring to detect the displacement through strain [38–40].However, the cumulative strain calculation displacement causes a certain error. Based onthe aforementioned limitations of the previous studies, this study considers the YuanbaoBay 6107 longwall face [2,3,41] as the study location in order to investigate the stabilityof the coal pillar in undermining conditions using the adopted experimental method. Byadopting the three key strata shift measurement point layout, this experiment captured

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the violent settlement caused by strong ground pressure, then studied the mechanismof the dynamic collapse of the coal pillar group through physical simulation, and finally,deeply analyzed the mechanism of the disaster through a simplified model. This researchachieved the goal of monitoring the movement characteristics [42] of the overlying strataand studying the disaster-causing mechanism of strong ground pressure as well as mon-itoring the movement characteristics of the overlying strata and studying the disastermechanism caused by strong ground pressure. Moreover, this study represents an impor-tant contribution to the prevention and control of support crushing disasters by puttingforward effective measures and countermeasures to guarantee the safety of mine personnel,equipment, and facilities by predicting the initial location of an occurrence of strong groundpressure and carrying out effective prevention and control through methods like hydraulicfracturing to solve the vulnerabilities in advance.

2. Basic Conditions of Longwall Face

There are four key strata in the overburden of the 6107 longwall face, all of which aremedium sandstone, based on the borehole columnar section 2101. From bottom to top, thelowest region has a burial depth of 148.9 m and a thickness of 7.75 m, which exist in theinferior key stratum between the No. 6 and No. 4 coal seams. The next layer has a burialdepth of 126.3 m and an inferior key stratum thickness of 8.8 m. Then, the succeeding layerhas a burial depth of 76.7 m, and the thickness of the inferior key stratum is 5.5 m. Finally,the main key stratum has a burial depth of 49.5 m and a thickness of 10.5 m.

The mining method adopted in this study is one-time full-height longwall retreatingmining with the 6107 longwall face having an advancing length of 500 m, a width of240 m, an average coal seam thickness of 3.5 m, an inclination angle of 4◦ to 8◦ (with anaverage of 6◦), and a coal seam burial depth of approximately 150 m. The No. 6 coalseam is overlaid by the room-and-pillar mining goaf formed by small coal kilns beforeintegrating No. 3 and No. 4 coal seams. There is still a certain bearing capacity based onthe preliminary exploration. Combining the actual survey results on the site, the coal pillarwidths of No. 3 and No. 4 coal seams are approximately 5–7 m, and the goaf of them areabout 4-6 m.

The main problem with mining in the 6107 longwall face of Yuanbao Bay Mine is thethreat of collapse of the coal pillars in the room-and-pillar mined-out area in the overlying3 and 4 coal goaf. If the overlying goaf is affected by the undermining of the No. 6 coal seamlongwall faces, it will cause a large-scale collapse that will result in the overall breakingmovement of the overburden, inducing the occurrence of dynamic loading ground pressuredisasters on the longwall face.

3. Characteristics of Strong Ground Pressure3.1. Location and Situation of Strong Ground Pressure

A strong ground pressure event occurred on August 12. The head and tail positionsof the 6107 longwall face were advanced to 318 and 325 m (with an average distance of322 m), respectively. The pressure on support Nos. 16 to 135 exceeded 40 MPa, and allsafety valves were opened. Among them, support Nos. 110 to 135 and Nos. 80 to 110 hada stroke of support movable column of 60 and 40 cm, respectively. In addition, supportNos. 16 to 80 had no stroke. The supports were crushed, and the advance of the longwallface was stopped for several days. Figure 1 shows the range of the longwall face supportcrushing and the position of the newly developed fractures [43].

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Figure 1. Plan view of the position of the newly developed fractures and the range of longwall face support crushing.

On the day of the occurrence of strong ground pressure, a detailed survey of the de-velopment of the surface fractures was carried out. According to the survey results, it was found that two groups of newly developed surface fractures appeared at the position, 50 m in front of the longwall face, and each group of fractures consisted of 3–4 stepped frac-tures, as shown in Figure 1. The first group of fractures was approximately located near holes No. 1 and No. 2, and the second group was approximately located near hole No. 10. The direction of the fractures was roughly along the longwall face advancing direction. The surface fractures near hole No. 2 developed as shown in Figure 2. The maximum width of these surface fractures was 1.0 m, and the maximum displacement of the steps was 0.8 m.

Figure 2. Surface fracture development near hole No. 2.

3.2. Characteristics of Unstable Overburden Movement under Strong Ground Pressure To further understand the characteristics of the overburden movement, it was de-

cided to arrange multiple strata movement measuring points at the key stratum position as required. A strong ground pressure phenomenon was observed during the monitoring, and the strata movement characteristics when strong ground pressure occurred were cap-tured. Holes No. 6 and 10 were chosen as the internal strata displacement installation holes based on the results of the borehole columnar section, key stratum identification, and the investigation results of the longwall face drill hole. Specifically, the position of hole No. 6 was in the center of the range of the newly developed fractures and used to illustrate the overburden movement process, the No. 6 drill hole installation plan, as is shown in Figure 3a, and the remaining holes (i.e., 1, 2, 3, 5, and 13) of grouting used for the borescope peeping.

Figure 1. Plan view of the position of the newly developed fractures and the range of longwall facesupport crushing.

On the day of the occurrence of strong ground pressure, a detailed survey of thedevelopment of the surface fractures was carried out. According to the survey results, itwas found that two groups of newly developed surface fractures appeared at the position,50 m in front of the longwall face, and each group of fractures consisted of 3–4 steppedfractures, as shown in Figure 1. The first group of fractures was approximately located nearholes No. 1 and No. 2, and the second group was approximately located near hole No. 10.The direction of the fractures was roughly along the longwall face advancing direction. Thesurface fractures near hole No. 2 developed as shown in Figure 2. The maximum width ofthese surface fractures was 1.0 m, and the maximum displacement of the steps was 0.8 m.

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Figure 1. Plan view of the position of the newly developed fractures and the range of longwall face support crushing.

On the day of the occurrence of strong ground pressure, a detailed survey of the de-velopment of the surface fractures was carried out. According to the survey results, it was found that two groups of newly developed surface fractures appeared at the position, 50 m in front of the longwall face, and each group of fractures consisted of 3–4 stepped frac-tures, as shown in Figure 1. The first group of fractures was approximately located near holes No. 1 and No. 2, and the second group was approximately located near hole No. 10. The direction of the fractures was roughly along the longwall face advancing direction. The surface fractures near hole No. 2 developed as shown in Figure 2. The maximum width of these surface fractures was 1.0 m, and the maximum displacement of the steps was 0.8 m.

Figure 2. Surface fracture development near hole No. 2.

3.2. Characteristics of Unstable Overburden Movement under Strong Ground Pressure To further understand the characteristics of the overburden movement, it was de-

cided to arrange multiple strata movement measuring points at the key stratum position as required. A strong ground pressure phenomenon was observed during the monitoring, and the strata movement characteristics when strong ground pressure occurred were cap-tured. Holes No. 6 and 10 were chosen as the internal strata displacement installation holes based on the results of the borehole columnar section, key stratum identification, and the investigation results of the longwall face drill hole. Specifically, the position of hole No. 6 was in the center of the range of the newly developed fractures and used to illustrate the overburden movement process, the No. 6 drill hole installation plan, as is shown in Figure 3a, and the remaining holes (i.e., 1, 2, 3, 5, and 13) of grouting used for the borescope peeping.

Figure 2. Surface fracture development near hole No. 2.

3.2. Characteristics of Unstable Overburden Movement under Strong Ground Pressure

To further understand the characteristics of the overburden movement, it was decidedto arrange multiple strata movement measuring points at the key stratum position asrequired. A strong ground pressure phenomenon was observed during the monitoring,and the strata movement characteristics when strong ground pressure occurred werecaptured. Holes No. 6 and 10 were chosen as the internal strata displacement installationholes based on the results of the borehole columnar section, key stratum identification,and the investigation results of the longwall face drill hole. Specifically, the position ofhole No. 6 was in the center of the range of the newly developed fractures and used toillustrate the overburden movement process, the No. 6 drill hole installation plan, as isshown in Figure 3a, and the remaining holes (i.e., 1, 2, 3, 5, and 13) of grouting used for theborescope peeping.

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Figure 3. Internal strata movement installation plan of the longwall face. (a) Installation plan of the internal strata move-ment measurement and observation holes; (b) Diagram of the internal strata movement measurement hole No. 6 installa-tion.

A schematic diagram of the internal strata movement measurement hole No. 6 instal-lation is shown in Figure 3b, where each measuring point represents the movement state of each key stratum and its controlled strata. The alternative relationship between each measuring point sinking amount and the distance between the 6107 longwall face and hole No. 6 is shown in Figure 4.

Figure 4. Relationship between each measuring point sinking amount and the distance between the 6107 longwall face and hole No. 6.

Figure 4 shows a large sinking amount of measuring points 6-1 and 6-2 with sinking increments of 70 and 30 mm, fast change process, and short and rapid time when the longwall face is pushed through the hole for about 8 m, indicating that the first step sub-sidence occurred in the strata corresponding to the two measuring points that are in the active period, which is short, of the overburden movement. All measuring points change slowly for a long period when the longwall face is pushed through the hole at approxi-mately 28 m. The total increments of its subsidence were 120, 80, and 40 mm, with the relative increments of 50, 50, and 40 mm, respectively. This indicates that the strata corre-sponding to the three measurement points had secondary step subsidence with a similar

Figure 3. Internal strata movement installation plan of the longwall face. (a) Installation plan of theinternal strata movement measurement and observation holes; (b) Diagram of the internal stratamovement measurement hole No. 6 installation.

A schematic diagram of the internal strata movement measurement hole No. 6installation is shown in Figure 3b, where each measuring point represents the movementstate of each key stratum and its controlled strata. The alternative relationship betweeneach measuring point sinking amount and the distance between the 6107 longwall face andhole No. 6 is shown in Figure 4.

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Figure 3. Internal strata movement installation plan of the longwall face. (a) Installation plan of the internal strata move-ment measurement and observation holes; (b) Diagram of the internal strata movement measurement hole No. 6 installa-tion.

A schematic diagram of the internal strata movement measurement hole No. 6 instal-lation is shown in Figure 3b, where each measuring point represents the movement state of each key stratum and its controlled strata. The alternative relationship between each measuring point sinking amount and the distance between the 6107 longwall face and hole No. 6 is shown in Figure 4.

Figure 4. Relationship between each measuring point sinking amount and the distance between the 6107 longwall face and hole No. 6.

Figure 4 shows a large sinking amount of measuring points 6-1 and 6-2 with sinking increments of 70 and 30 mm, fast change process, and short and rapid time when the longwall face is pushed through the hole for about 8 m, indicating that the first step sub-sidence occurred in the strata corresponding to the two measuring points that are in the active period, which is short, of the overburden movement. All measuring points change slowly for a long period when the longwall face is pushed through the hole at approxi-mately 28 m. The total increments of its subsidence were 120, 80, and 40 mm, with the relative increments of 50, 50, and 40 mm, respectively. This indicates that the strata corre-sponding to the three measurement points had secondary step subsidence with a similar

Figure 4. Relationship between each measuring point sinking amount and the distance between the6107 longwall face and hole No. 6.

Figure 4 shows a large sinking amount of measuring points 6-1 and 6-2 with sinkingincrements of 70 and 30 mm, fast change process, and short and rapid time when thelongwall face is pushed through the hole for about 8 m, indicating that the first stepsubsidence occurred in the strata corresponding to the two measuring points that are inthe active period, which is short, of the overburden movement. All measuring pointschange slowly for a long period when the longwall face is pushed through the hole atapproximately 28 m. The total increments of its subsidence were 120, 80, and 40 mm, withthe relative increments of 50, 50, and 40 mm, respectively. This indicates that the stratacorresponding to the three measurement points had secondary step subsidence with asimilar amount of subsidence, an active overburden movement, and a relatively longeractive period. After these two active periods, the sinking value did not change significantly.

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The ground pressure on the longwall face was extremely intense during the large-scalemovement of the overburden. In addition, the time and location of the sinking experienced byhole No. 6 had a one-to-one correspondence with the time and location of the strong groundpressure on the longwall face, indicating that the active period of the overburden movementand the occurrence of the strong ground pressure on the longwall face are inseparable.

3.3. Results of the Drill Hole Borescope Peeping

Borehole No. 1 is within the range of newly developed fractures, reflecting the detailedprocess of the overlying strata movement, as shown in Figure 5.

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amount of subsidence, an active overburden movement, and a relatively longer active pe-riod. After these two active periods, the sinking value did not change significantly.

The ground pressure on the longwall face was extremely intense during the large-scale movement of the overburden. In addition, the time and location of the sinking expe-rienced by hole No. 6 had a one-to-one correspondence with the time and location of the strong ground pressure on the longwall face, indicating that the active period of the over-burden movement and the occurrence of the strong ground pressure on the longwall face are inseparable.

3.3. Results of the Drill Hole Borescope Peeping Borehole No. 1 is within the range of newly developed fractures, reflecting the de-

tailed process of the overlying strata movement, as shown in Figure 5.

Figure 5. Observation results of borehole No. 1. (a) Water accumulation location at −64.46 m; Error hole position at (b) −94.86 m, (c) −88.61 m, (d) −78.82 m, (e) −73.17 m, and (f) −35.20 m.

The observations started when borehole No. 1 was 53 m away from the longwall face. Results from the observations show that the probe was lowered to −64.46 m, wherein wa-ter accumulated in the hole, and cracks under the hole did not develop. Therefore, the

Figure 5. Observation results of borehole No. 1. (a) Water accumulation location at −64.46 m; Errorhole position at (b) −94.86 m, (c) −88.61 m, (d) −78.82 m, (e) −73.17 m, and (f) −35.20 m.

The observations started when borehole No. 1 was 53 m away from the longwallface. Results from the observations show that the probe was lowered to −64.46 m, whereinwater accumulated in the hole, and cracks under the hole did not develop. Therefore,the probe was stopped, as shown in Figure 5a. Additionally, the probe was lowered toapproximately −94.86 m when the longwall face was 20 m from the hole. Error holes inthe hole were found, and the distance between them was relatively large, as shown in

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Figure 5b. Moreover, the position of the staggered hole in the hole remained unchangedat −94.86 m when the longwall face was 17 to 0 m. We found error holes when the probewas lowered to a position of approximately −88.61 m, and the longwall face was pushedthrough the hole at 5 m. The distance of the error holes was not large, but the probe wasnot able to pass, and the descending was forced to stop, as shown in Figure 5c. Duringthe 6 to 27 m period when the longwall face pushed through the hole, the position of theerror hole did not change. When the longwall face was pushed through 33 m, it was foundthat when the probe was lowered to about −78.82 m, the drill hole was slightly wrong, thecasing fell off, the probe could not pass, and the detection ended, as shown in Figure 5d.The position of the error hole remained unchanged from 39 to 43 m when the longwall facewas pushed through the borehole. The probe went down to approximately −73.17 m, andthe error hole occurred when the longwall face was pushed through the borehole for morethan 48 m. The error hole was not completely closed, but the probe could not pass through,so it stopped descending, as shown in Figure 5e. In the 53–58 m period, the position of theerror hole remained unchanged when the longwall face was pushed through the borehole.On the other hand, the probe was lowered to −35.20 m, and the error hole occurred whenthe longwall face was pushed through the hole at 64 m, as shown in Figure 5f. It was oflittle use to continue the detection because the position of the error hole was already abovethe main key stratum. Thus, the borescope observation was complete.

According to the above observation results, the relationship of the depth and therelative distance between the longwall face and hole No. 1 and the position of the staggeredhole can be obtained, as shown in Figure 6. The first misalignment of the hole coincideswith the location of the strong ground pressure, indicating that the large movement ofthe overlying strata below the inferior key stratum is the main cause of the strong groundpressure. In the subsequent mining engineering of the longwall face, the position of theerror hole has a small step rise, and finally, the position of the error hole near the loose layerno longer changes. The observation results indicate that the entire overburden movementhas a stepwise upward trend.

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probe was stopped, as shown in Figure 5a. Additionally, the probe was lowered to ap-proximately −94.86 m when the longwall face was 20 m from the hole. Error holes in the hole were found, and the distance between them was relatively large, as shown in Figure 5b. Moreover, the position of the staggered hole in the hole remained unchanged at −94.86 m when the longwall face was 17 to 0 m. We found error holes when the probe was low-ered to a position of approximately −88.61 m, and the longwall face was pushed through the hole at 5 m. The distance of the error holes was not large, but the probe was not able to pass, and the descending was forced to stop, as shown in Figure 5c. During the 6 to 27 m period when the longwall face pushed through the hole, the position of the error hole did not change. When the longwall face was pushed through 33 m, it was found that when the probe was lowered to about −78.82 m, the drill hole was slightly wrong, the casing fell off, the probe could not pass, and the detection ended, as shown in Figure 5d. The position of the error hole remained unchanged from 39 to 43 m when the longwall face was pushed through the borehole. The probe went down to approximately −73.17 m, and the error hole occurred when the longwall face was pushed through the borehole for more than 48 m. The error hole was not completely closed, but the probe could not pass through, so it stopped descending, as shown in Figure 5e. In the 53–58 m period, the position of the error hole remained unchanged when the longwall face was pushed through the borehole. On the other hand, the probe was lowered to −35.20 m, and the error hole occurred when the longwall face was pushed through the hole at 64 m, as shown in Figure 5f. It was of little use to continue the detection because the position of the error hole was already above the main key stratum. Thus, the borescope observation was complete.

According to the above observation results, the relationship of the depth and the rel-ative distance between the longwall face and hole No. 1 and the position of the staggered hole can be obtained, as shown in Figure 6. The first misalignment of the hole coincides with the location of the strong ground pressure, indicating that the large movement of the overlying strata below the inferior key stratum is the main cause of the strong ground pressure. In the subsequent mining engineering of the longwall face, the position of the error hole has a small step rise, and finally, the position of the error hole near the loose layer no longer changes. The observation results indicate that the entire overburden movement has a stepwise upward trend.

Figure 6. Relationship between the depth of misaligned holes in No. 1 borehole and the distance of longwall face relative to No. 1 borehole.

Figure 6. Relationship between the depth of misaligned holes in No. 1 borehole and the distance oflongwall face relative to No. 1 borehole.

4. Dynamic Collapse Process of Room Pillars and the Disaster Mechanism caused byStrong Ground Pressure4.1. Scheme Design of the Similar Simulation Model

The occurrence of the strong ground pressure disaster on the 6107 longwall face wascaused by the overall movement of the overburden. Therefore, we employed a similarsimulation [44] to study the disaster mechanism of the strong ground pressure on the6107 longwall face of Yuanbao Bay Mine caused by the overlying room pillar collapse.

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The purpose of this simulation is: (1) to simulate the overlying room pillar dynamiccollapse process on the 6107 longwall face; (2) to simulate the movement characteristics ofthe overlying strata after the coal pillar collapse; and (3) to analyze the disaster mechanismbased on the collapse process and the characteristics of the overlying strata movement.

We selected a two-dimensional model frame with a dimension of 1.3 m × 0.12 m × 1 mfor the simulation based on the mining conditions and occurrence characteristics of thelongwall face. Based on the same conditions, the geometric similarity ratio of the modelwas 1:100, Poisson’s ratio similarity ratio was 1:1, density similarity ratio was 1:1.6, andstress similarity ratio was 1:160. The height of the model was 65 cm. A correspondingload of approximately 80 m thickness overlying rock was applied to simulate the actualburial depth, equivalently replaced by a uniform load of 12.5 kPa according to the stresssimilarity ratio. A schematic of the model is shown in Figure 7.

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4. Dynamic Collapse Process of Room Pillars and the Disaster Mechanism caused by Strong Ground Pressure 4.1. Scheme Design of the Similar Simulation Model

The occurrence of the strong ground pressure disaster on the 6107 longwall face was caused by the overall movement of the overburden. Therefore, we employed a similar simulation [44] to study the disaster mechanism of the strong ground pressure on the 6107 longwall face of Yuanbao Bay Mine caused by the overlying room pillar collapse.

The purpose of this simulation is: (1) to simulate the overlying room pillar dynamic collapse process on the 6107 longwall face; (2) to simulate the movement characteristics of the overlying strata after the coal pillar collapse; and (3) to analyze the disaster mechanism based on the collapse process and the characteristics of the overlying strata movement.

We selected a two-dimensional model frame with a dimension of 1.3 m × 0.12 m × 1 m for the simulation based on the mining conditions and occurrence characteristics of the longwall face. Based on the same conditions, the geometric similarity ratio of the model was 1:100, Poisson’s ratio similarity ratio was 1:1, density similarity ratio was 1:1.6, and stress similarity ratio was 1:160. The height of the model was 65 cm. A corresponding load of approximately 80 m thickness overlying rock was applied to simulate the actual burial depth, equivalently replaced by a uniform load of 12.5 kPa according to the stress similar-ity ratio. A schematic of the model is shown in Figure 7.

Figure 7. Schematic diagram of similar simulation model.

There are three strata for the overburden according to the borehole columnar section 2101 and the key stratum identification results. The number of key strata was also set to three layers, and the others were replaced by soft rocks whose structure is the interbedded mudstone and sandy mudstone in the bedrock. The mechanical properties of these two rock formations are very similar, so they can be approximately reckoned as the same ma-terial to simplify the structure of the overlying rock and reflect the main control effect of the key strata [6]. The thickness of each layer is 1.0–3.5 m; after homogenization, each layer thickness is 2 cm, as shown in Table 1.

Observation marks were placed on the roof of each coal seam to understand the sub-sidence of the coal roof in detail. There were 14 measuring points in each roof line with a 10 cm interval between them.

Figure 7. Schematic diagram of similar simulation model.

There are three strata for the overburden according to the borehole columnar section2101 and the key stratum identification results. The number of key strata was also set tothree layers, and the others were replaced by soft rocks whose structure is the interbeddedmudstone and sandy mudstone in the bedrock. The mechanical properties of these two rockformations are very similar, so they can be approximately reckoned as the same materialto simplify the structure of the overlying rock and reflect the main control effect of thekey strata [6]. The thickness of each layer is 1.0–3.5 m; after homogenization, each layerthickness is 2 cm, as shown in Table 1.

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Table 1. Proportion table of similar materials.

No. Lithology M

Material/kg

t SSand Calcium

Carbonate Plaster Water

11 Soft rock 773 21.84 2.18 0.94 2.78 10 510 KS3 337 10.92 1.09 2.55 1.62 5 19 Soft rock 773 21.84 2.18 0.94 2.78 10 58 No. 3 coal seam 773 8.74 0.87 0.37 1.08 4 17 Soft rock 773 8.74 0.87 0.37 1.11 4 26 KS2 437 15.29 1.15 2.68 2.12 7 15 No. 4 coal seam 773 13.10 1.31 0.56 1.61 6 14 Soft rock 773 13.10 1.31 0.56 1.66 6 33 KS1 437 10.92 0.82 1.91 1.52 5 12 No. 6 coal seam 773 8.74 0.87 0.37 1.08 4 11 Floor 673 8.74 1.02 0.44 1.13 4 2

Note: t is the thickness of strata, m; S is the stratification of strata; M is the matching number of material.

Observation marks were placed on the roof of each coal seam to understand thesubsidence of the coal roof in detail. There were 14 measuring points in each roof line witha 10 cm interval between them.

The goaf areas of the No. 3 and No. 4 coal seams were designed according to theactual engineering background ratios of mined to unmined width, 1.8 and 2.8, respectively.Generally considered, the widths of the No. 3 coal seam goaf and the coal pillar were 7 and3 cm, respectively. Meanwhile, the widths of the No. 4 coal seam goaf and the coal pillarwere 8 and 2 cm, respectively. Both 6.5 and 6 m width of coal pillars were left on the modelboundaries of the No. 3 and No. 4 coal seams, respectively, to avoid the influence of themodel boundary on the goaf during the mining period.

The No. 6 coal seam mining length was 90 cm, with 10 cm width boundary coal pillarsleft on both sides. It had a mining step of 5 cm, a total of 18 excavations, and the timeinterval between each excavation of half an hour was used for the strata to stabilize. Then,we employed high-speed and real-time photography.

4.2. Dynamic Collapse Process of Room Pillars

The state of the No. 6 coal seam roof changes significantly, eventually breaks, and theamount of subsidence increases significantly up to 45 mm when the longwall face advances70 cm. Then, the advancing distance continues to increase, while the maximum subsidencevalue of the roof of the No. 6 coal seam no longer increased. However, the lateral range ofits movement subsidence increases from 90 to 120 cm, as shown in Figure 8a. Results showthat the No. 4 coal seam roof sinks later than the No. 6 coal seam. The No. 4 coal seam roofdrastically sinks and breaks after the coal pillar becomes unstable at 70 cm, and the sinkingamount suddenly increases to approximately 70 mm. The amount of coal roof subsidenceremains unchanged as the longwall face continuously advances, while the subsidencerange increased slightly, as shown in Figure 8b. Moreover, the amount of roof subsidenceof the No. 3 coal seam increases when the advancing distance is within 60 cm. On the otherhand, the advancing distance of the longwall face is 70 cm when the coal roof breaks andsinks, which is the same position as the roof-breaking movement of the No. 6 and No. 4coal seams. The amount of subsidence increases up to approximately 90 cm, indicating thatthe roof of the No. 3 coal seam moves violently after the overburden movement. Moreover,subsequent mining has little effect on the sinking movement of the No. 3 coal seam roof, asshown in Figure 8c.

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(a) (b)

(c)

Figure 8. Coal seam roof subsidence of (a) No.6, (b) No.4, and (c) No.3 during model excavation.

Figure 9 shows the failure sequence and the detailed process of the dynamic collapse of coal pillars of overlying No. 3 and No. 4 coal seams on the longwall face. Additionally, the overlying coal pillars of the No. 3 and No. 4 coal seam rooms and pillar goaf were affected by the mining process of the longwall face, resulting in these two being succes-sively damaged and failing. This caused the large-scale collapse of the two layers of coal pillars, resulting in the occurrence of strong ground pressure on the longwall face.

Both the No. 3 and No. 4 coal seam pillars, which consist of 11 coal pillars from No. 1 to No. 11, are numbered from right to left, as shown in Figure 9a. The first failure oc-curred at No. 4, No. 5, and No. 11 coal pillars of the No. 4 coal seam. Moreover, the No. 6 coal seam immediate, the controlled strata broke and revolved, and the No. 4 coal seam roof strata bent and sank. Fractures developed directly on the coal seam roof at the posi-tion of the setup entry and the No. 6 coal seam longwall face, as shown in Figure 9b. Even-tually, the No. 4 and No. 5 coal pillars of the No. 3 coal seam failed, and fractures devel-oped directly on the boundary of the coal pillar of the No. 4 coal seam, as shown in Figure 9c. Additionally, the No. 8 and No. 9 coal pillars of the No. 4 coal seam and No. 3 coal pillar of the No. 3 coal seam successively failed, the immediate roof of the No. 4 coal seam inferior key stratum and its controlled strata broke and rotated, and the No. 3 coal seam overlying strata was bent and subsided, as shown in Figure 9d. Overall, the coal pillars of the No. 4 coal seam above the goaf are all unstable, and the adjacent coal pillars that failed in the No. 3 coal seam have a large number of fractures in the roof, as shown in Figure 9e. The coal pillar of the No. 3 coal seam suffered a large-scale collapse, and the overlying roof strata broke and turned, causing the interburden of No. 3 and No. 4 coal seams to be entirely cut off. Moreover, the immediate roof of the No. 4 coal seam acted on the inter-burden of the No. 4 and No. 6 coal seams to cut instantaneously and fall down, causing the longwall face support crushing, as shown in Figure 9f.

0 10 20 30 40 50 60 70 80 90

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Figure 8. Coal seam roof subsidence of (a) No. 6, (b) No. 4, and (c) No. 3 during model excavation.

Figure 9 shows the failure sequence and the detailed process of the dynamic collapseof coal pillars of overlying No. 3 and No. 4 coal seams on the longwall face. Additionally,the overlying coal pillars of the No. 3 and No. 4 coal seam rooms and pillar goaf wereaffected by the mining process of the longwall face, resulting in these two being successivelydamaged and failing. This caused the large-scale collapse of the two layers of coal pillars,resulting in the occurrence of strong ground pressure on the longwall face.

Both the No. 3 and No. 4 coal seam pillars, which consist of 11 coal pillars from No. 1to No. 11, are numbered from right to left, as shown in Figure 9a. The first failure occurredat No. 4, No. 5, and No. 11 coal pillars of the No. 4 coal seam. Moreover, the No. 6 coalseam immediate, the controlled strata broke and revolved, and the No. 4 coal seam roofstrata bent and sank. Fractures developed directly on the coal seam roof at the position ofthe setup entry and the No. 6 coal seam longwall face, as shown in Figure 9b. Eventually,the No. 4 and No. 5 coal pillars of the No. 3 coal seam failed, and fractures developeddirectly on the boundary of the coal pillar of the No. 4 coal seam, as shown in Figure 9c.Additionally, the No. 8 and No. 9 coal pillars of the No. 4 coal seam and No. 3 coal pillar ofthe No. 3 coal seam successively failed, the immediate roof of the No. 4 coal seam inferiorkey stratum and its controlled strata broke and rotated, and the No. 3 coal seam overlyingstrata was bent and subsided, as shown in Figure 9d. Overall, the coal pillars of the No. 4coal seam above the goaf are all unstable, and the adjacent coal pillars that failed in the No.3 coal seam have a large number of fractures in the roof, as shown in Figure 9e. The coalpillar of the No. 3 coal seam suffered a large-scale collapse, and the overlying roof stratabroke and turned, causing the interburden of No. 3 and No. 4 coal seams to be entirely cutoff. Moreover, the immediate roof of the No. 4 coal seam acted on the interburden of theNo. 4 and No. 6 coal seams to cut instantaneously and fall down, causing the longwall facesupport crushing, as shown in Figure 9f.

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Figure 9. Dynamic collapse process of room-and-pillar mining pillars. (a) Before the coal pillar loses stability; (b) No.4, No.5, and No.11 coal pillars fail first; (c) No.4 and No.5 coal pillars fail successively; (d) No.8 and No.9 coal pillars fail; (e) Large area of coal pillar collapse; (f) Support crushing under strong ground pressure at longwall face.

4.3. Analysis of the Disaster Mechanism of the Strong Ground Pressure We obtained a model of the dynamic collapse of the coal pillar disaster mechanism

based on the aforementioned process, as shown in Figure 10. The immediate roof of the longwall face is a hard, inferior key stratum. The longwall

face is affected by the advancing abutment pressure [45–47] when it advances to form a larger span. The diagonal failure of the coal pillars of the No. 4 coal seam above the setup entry and longwall face occurs first because the peak value of the advancing abutment pressure is larger than the compressive strength of the coal pillar, as shown in Figure 10a. The stress from the diagonal failure was instantly transferred to the pillars above the goaf, resulting in the breakage and rotation of the immediate roof that was exposed in the goaf of the longwall face of the No. 6 coal seam due to the sudden increase in stress. Addition-ally, the space of the immediate roof rotation was limited by the mining height, which forced the rotation to stop. Specifically, the No. 4 coal seam pillars overlying the mined-out area were subjected to excessive overburden load and collapsed. After this large-scale failure, the immediate roof also stopped moving in the limited space. On the other hand, the No. 3 coal seam pillars overlying the setup entry and longwall face were also diago-nally damaged during the movement of the strata below, as shown in Figure 10b. Moreo-ver, the advancing abutment pressure was continuously being transferred to the undam-aged coal pillars of the No. 3 coal seam, resulting in the advanced coal pillar failure. The load of the No.3 coal seam was concentrated on the coal pillars that had not failed in the middle. The coal pillars were destroyed and failed in a larger area due to the significant pressure from the overload. The inferior key stratum of each coal seam and the controlled strata developed through fractures. The No. 3 coal seam roof strata also tended to move

Figure 9. Dynamic collapse process of room-and-pillar mining pillars. (a) Before the coal pillar loses stability; (b) No. 4,No. 5, and No. 11 coal pillars fail first; (c) No. 4 and No. 5 coal pillars fail successively; (d) No. 8 and No. 9 coal pillars fail;(e) Large area of coal pillar collapse; (f) Support crushing under strong ground pressure at longwall face.

4.3. Analysis of the Disaster Mechanism of the Strong Ground Pressure

We obtained a model of the dynamic collapse of the coal pillar disaster mechanismbased on the aforementioned process, as shown in Figure 10.

The immediate roof of the longwall face is a hard, inferior key stratum. The longwallface is affected by the advancing abutment pressure [45–47] when it advances to form alarger span. The diagonal failure of the coal pillars of the No. 4 coal seam above the setupentry and longwall face occurs first because the peak value of the advancing abutmentpressure is larger than the compressive strength of the coal pillar, as shown in Figure 10a.The stress from the diagonal failure was instantly transferred to the pillars above the goaf,resulting in the breakage and rotation of the immediate roof that was exposed in the goaf ofthe longwall face of the No. 6 coal seam due to the sudden increase in stress. Additionally,the space of the immediate roof rotation was limited by the mining height, which forced therotation to stop. Specifically, the No. 4 coal seam pillars overlying the mined-out area weresubjected to excessive overburden load and collapsed. After this large-scale failure, theimmediate roof also stopped moving in the limited space. On the other hand, the No. 3 coalseam pillars overlying the setup entry and longwall face were also diagonally damagedduring the movement of the strata below, as shown in Figure 10b. Moreover, the advancingabutment pressure was continuously being transferred to the undamaged coal pillars ofthe No. 3 coal seam, resulting in the advanced coal pillar failure. The load of the No. 3 coalseam was concentrated on the coal pillars that had not failed in the middle. The coal pillarswere destroyed and failed in a larger area due to the significant pressure from the overload.

Energies 2021, 14, 5221 12 of 15

The inferior key stratum of each coal seam and the controlled strata developed throughfractures. The No. 3 coal seam roof strata also tended to move instantaneously because ofthe loss of the coal pillar support, as shown in Figure 10c. Moreover, the overlying strataof the No. 3 coal seam were entirely cut off along the through fractures of the overlyingstrata after the pillars of the No. 3 coal seam suffered a large area of collapse because therewas no coal pillar support. This directly impacted the underlying strata of the No. 3 coalseam, and eventually, the interburden of the No. 3 and No. 4 coal seams were entirely cutoff along its through fractures. The two huge impacts formed by the overall cutting of thetwo parts of the No. 4 coal seam overlying strata were superimposed on each other, actingon the interburden of the No. 4 and No. 6 coal seams. The violent impact made it instantlycut down along the entire intersecting fractures, directly acting on the longwall face thatcaused a strong ground pressure and caused the longwall face support crushing, as shownin Figure 10d.

Energies 2021, 14, x FOR PEER REVIEW 12 of 15

instantaneously because of the loss of the coal pillar support, as shown in Figure 10c. Moreover, the overlying strata of the No. 3 coal seam were entirely cut off along the through fractures of the overlying strata after the pillars of the No.3 coal seam suffered a large area of collapse because there was no coal pillar support. This directly impacted the underlying strata of the No. 3 coal seam, and eventually, the interburden of the No. 3 and No. 4 coal seams were entirely cut off along its through fractures. The two huge impacts formed by the overall cutting of the two parts of the No. 4 coal seam overlying strata were superimposed on each other, acting on the interburden of the No. 4 and No. 6 coal seams. The violent impact made it instantly cut down along the entire intersecting fractures, di-rectly acting on the longwall face that caused a strong ground pressure and caused the longwall face support crushing, as shown in Figure 10d.

Figure 10. Diagram of disaster mechanism caused by dynamic collapse of coal pillars. (a) Diagonal failure of pillars of the No.4 coal seam first occurs; (b) Continuous shear failure of adjacent coal pillars; (c) Local overall failure of pillars of No.3 coal seam; (d) Whole-rock strata cut to strong ground.

5. Conclusions In this study, the collapse and fracture of the coal pillar group and the movement

characteristics of the overlying strata in the undermining process employed at the Yu-anbao Bay 6107 longwall face are determined and investigated. The principles caused by the dynamic collapse of the coal pillar and the disaster mechanism of the strong ground pressure are identified and investigated using strata movement monitoring, borehole peeping, similar simulation experiments, and simplified model deepening analysis. The main conclusions of this study are as follows: (1) Results show that the overlying strata movement presents the movement characteris-

tics of the entire step subsidence, while the staggered hole position of each borehole shows the characteristics of the stage step rise. Particularly, the strata movement data of each measuring point in boreholes No. 6 and No. 10 significantly increased when the strong ground pressure occurred. Simultaneously, the staggered position of each borehole moves upward, which is consistent with the time and position of the occur-rence of the strong ground pressure on the 6107 longwall face. Moreover, the collapse

Figure 10. Diagram of disaster mechanism caused by dynamic collapse of coal pillars. (a) Diagonalfailure of pillars of the No. 4 coal seam first occurs; (b) Continuous shear failure of adjacent coal pillars;(c) Local overall failure of pillars of No. 3 coal seam; (d) Whole-rock strata cut to strong ground.

5. Conclusions

In this study, the collapse and fracture of the coal pillar group and the movementcharacteristics of the overlying strata in the undermining process employed at the YuanbaoBay 6107 longwall face are determined and investigated. The principles caused by thedynamic collapse of the coal pillar and the disaster mechanism of the strong groundpressure are identified and investigated using strata movement monitoring, boreholepeeping, similar simulation experiments, and simplified model deepening analysis. Themain conclusions of this study are as follows:

(1) Results show that the overlying strata movement presents the movement characteris-tics of the entire step subsidence, while the staggered hole position of each boreholeshows the characteristics of the stage step rise. Particularly, the strata movementdata of each measuring point in boreholes No. 6 and No. 10 significantly increasedwhen the strong ground pressure occurred. Simultaneously, the staggered position ofeach borehole moves upward, which is consistent with the time and position of theoccurrence of the strong ground pressure on the 6107 longwall face. Moreover, the

Energies 2021, 14, 5221 13 of 15

collapse of the coal pillar left by the No. 4 coal seam leads to a strong ground pressurephenomenon on the 6107 longwall face.

(2) Without undermining, the No. 3 and No. 4 coal pillars, which are affected by theirsizes, can maintain long-term stability. Meanwhile, in undermining conditions, theNo. 4 coal pillars were found to be unable to maintain stability caused by the initialcoal pillar collapse due to the influence of the advancing abutment pressure, whilethe No. 3 coal pillars can maintain stability.

(3) The overlying coal pillars of the No. 4 coal seam were first diagonally damagedat both ends above the goaf, and then the coal pillar above the middle of the goafwas instantaneously invalidated because of the effect of mining of the longwall face.Moreover, the coal pillars of the No. 4 coal seam had a wide range of overall collapse.On the other hand, the coal pillars of the No. 3 coal seam at the corresponding positionof the goaf were diagonally damaged. Additionally, the pillars of the No. 3 coal seamwere destabilized on a large scale, wherein failure on both sides and in the middlewas apparent. Eventually, both pillars of the No. 3 and No. 4 coal seam experiencedlarge-scale collapse.

(4) The causes of the strong ground pressure on the longwall face are the overall largecutting movement of the overburden and the large area collapse of the coal pillar,which is influenced by the advancing abutment pressure. After the large-scale collapseof the coal pillar, the overlying strata has a large subsidence movement without thesupport of the coal pillar. This may cause the strata to be cut down entirely, directlyor indirectly impacting the longwall face, which causes a strong ground pressure onthe longwall face and support crushing.

Author Contributions: For this paper, W.Z. put forward study ideas; D.Y. designed the articlestructure and wrote the paper; Z.L. revised the whole English writing style and discussed the designflow with W.Z.; X.Y. collected and analyzed the data from mine site; J.L. conducted the similarsimulation experiment and analyzed the data. All authors have read and agreed to the publishedversion of the manuscript.

Funding: This research was funded by the National Natural Science Foundation of China (Grant No.52074265, No. 51874175).

Acknowledgments: The authors would like to thank the reviewers for their constructive comments,which improved the paper and anonymous colleagues for their kind efforts and valuable comments,which have improved this work.

Conflicts of Interest: The authors declare no conflict of interest.

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